A desirable membrane structure for use in the osmotic concentration of aqueous solutions via osmotic transfer of water to concentrated aqueous saline solutions, is a microporous, non-water-wettable membrane, contacted on its opposite surfaces by the solution to be concentrated and the brine into which the water is to be transferred. The membrane operates such that the pores thereof are filled only with water vapor, and water transport occurs only by molecular transport through the vapor phase. Membranes of such properties are well-known to those skilled in the membrane art, and include microporous structures fabricated from poly tetrafluorethylene, for example GORE-TEX (trade mark of W. L. Gore & Associates, Inc.), from polypropylene, for example CELGARD (Registered Trade Mark), ACCUREL (Registered Trade Mark) and the like.
In many instances, however, the proper functioning of such a membrane is prevented by the presence of contaminants or other components of the feed liquid which promote wetting of the membrane by that solution, and the subsequent penetration of the membrane pores by the wetting liquid. In such a situation, there may be leakage of the feed liquid into the brine-compartment, or of brine into the feed liquid.
Contaminants or other components present in the feed liquid which are likely to cause this problem include emulsified oil droplets, colloidal hydrogel particles, proteins and other biological macromolecules, and surface active agents. These components are, for the most part, non-volatile under the conditions of osmotic concentration, with the result that they remain behind in the feed liquid and may preferentially and adversely accumulate at the upstream membrane surface.
Components of solutions used to clean membrane devices often contain surface active agents, and that residual from these solutions, rather than feed components, might also promote membrane wetting.